High intensity discharge (HID) lamps include; for example, mercury vapor, metal halide and high-pressure sodium discharge lamps. These lamps are operated with ballast circuits to control the lamp operating current because of the negative voltage-current characteristics of the discharge arc within these lamps. Conventionally, electromagnetic transformer ballasts having a series connected inductance and capacitance (L-C) circuit in the form of a "choke" and a capacitor have been employed for this purpose.
Typically the ballast, HID lamp, and reflector are combined into a fixture or luminaire. For general illumination of, for example, warehouses and factories, a large number of luminaires are suspended from a ceiling, usually at spaced distances. Generally, a plurality of the luminaires are connected in an alternating (AC) power supply branch and controlled by a simple switch or circuit breaker which is adapted to switch all of the lamps between an "off" state, in which the lamps are completely extinguished, and an "on" state, in which the lamps are operated at full rated power.
It has become desirable in other types of lighting systems, for example, a fluorescent lighting system, to employ more sophisticated controls such as occupancy sensors to turn lamps off when nobody is present in a room and to turn the lights on when someone enters. However, this is not practical for HID lamps that typically require several minutes to ignite, warm-up and reach their full output levels. Additionally, most HID lamps have hot re-strike problems which make it difficult to re-ignite a lamp shortly after being turned off while they still remain at an elevated temperature. With some lamp-ballast combinations it may take up to approximately ten minutes after a lamp has been turned off before it will re-ignite. Employing a control system which turns HID lamps completely off is not feasible because the lamps will not provide sufficient light quickly enough if someone re-enters the space shortly thereafter. Moreover, a dimming condition is often desirable, in any event, to provide emergency lighting to the area serviced by the lamps. However, if HID lamps are operated at a reduced power level instead of being completely turned off they will return to a full or near full output power level within an acceptable period of time.
Numerous lighting control systems are known in the prior art, including power line carrier control systems, and those which employ a switched-capacitor method. Power line carrier (PLC) systems which use only the power line to connect the transmitter and receivers are well known for poor performance, especially with the occurrence of noise on the power line. Frequently, erroneous noise signals cause lights to change from one level to another. So, although PLC systems need no extra control line, the inclusion of the transmitter, receiver and phase coupler make them inherently complex and hence cost prohibitive for general bi-level illumination. A more acceptable control system for general bi-level HID illumination is the switched-capacitor control system. Fundamentally, switched-capacitor control systems place capacitors in parallel for high level power, and hence, high or bright illumination, and remove one capacitor for reduced power or dimmed illumination. An alternate method is to place the capacitors in series for low level power and short circuit one capacitor for high level power.
U.S. Pat. No. 4,931,701 (Carl) shows a bi-level control system which employs a switched-capacitor using a solid state zero-voltage-crossing relay as the switching mechanism. The zero-voltage-crossing relay was said to ensure that the switching-in (or switching-out) of the switched-capacitor is timed to occur at a zero-crossing point of the applied voltage. This applies or removes the switched-capacitor only when the voltage is not able to cause excessive voltage spikes or surges by switching the switched-capacitor if it is partly or fully charged when switched. Such partial or fill charge switching can cause damage to other components in the circuit. A disadvantage of such a solid state relay is that it allows a small current flow to the switched-capacitor unless the relay is specifically switched for dimming the lamp. The small current flow to the switched-capacitor has been found to cause unintentional dimming of the lamp from the full light output level. HID lamps inherently have a 5.degree. to 10.degree. phase lag, where the current lags the voltage. Hence, when voltage is zero across the switched-capacitor (in an HID lamp), there is still a small current flowing through the switched-capacitor. One object of the present invention is to utilize this inherent time delay to achieve capacitor switching during a zero current crossing. It has also been found that such a relay false triggers and closes at times other than zero-crossing of the input voltage to the lamp. Another disadvantage to Carl is that it uses a two-wire control circuit. Two-wire control circuits are an added installation expense that increases substantially when many luminaires are hung at a spaced distance. Such hanging configurations often occur in warehouses and factories.
U.S. Pat. No. 5,327,048 (Troy) attempts to alleviate some of the problems of a Carl-type control system. Troy teaches away from using zero-crossing relays. A specific objective of Troy is to protect ballast and relay components from current surges upon the switching out of switched-capacitor without employing solid state zero-crossing relays. Troy employs a ballast L-C circuit in series with the switched-capacitor. Troy also reduces the control circuit wiring. In Troy, one of the input lines of the capacitance switching means is connected to the common-line of the AC branch circuit and the other control line input is connected to the output of the control means with a single control line. Thus, the control means is effective to control the switching of said capacitance-switching means with a single control line by switching the source of electric potential to the output of the control means. Since the capacitance-switching means is connected to the same common line as the ballast and only a single control line connects the capacitance-switching means to the control, less wiring is needed than in Carl.
U.S. Pat. No. 5,451,843 (Kahn) also teaches away from the use of zero-crossing relays. Kahn shows a housing containing a control device comprised of a dual capacitor and a "random-crossing" relay connected to the capacitor. A single electrical control wire, similar to that in Troy is used. The control includes a switching relay of the random-crossing type. The relay is connected to and switches one of the capacitors into, or out of, the circuit. Such switching is irrespective of the instantaneous value of voltage across the switch terminals of the relay. The control device also includes an inductor, a terminal of which is connected to a capacitor. The inductor attenuates voltage "spikes" that can occur when the relay switches the charged capacitor in or out. Such voltage attenuation has the effect of limiting surge current.
It is desirable to develop a control device capable of switching the switched-capacitor when the current through said capacitor is zero or near zero. Such a device would reduce the need for inductor-capacitor "choke" surge protection components. Incorporating a simple Class 2 wiring scheme would further reduce installation costs for a plurality of luminaires hung at a spaced distance because additional conduit for the wiring of the control system would not be needed. Turning an HID lamp off during the warm up period of the lamp can potentially damage it. A method to prevent this potential damage would also be desirable.